This invention relates to a method for preparing tungsten ingots used in the manufacture of tungsten wire filaments for incandescent lamps.
It is known that the tungsten wire used in incandescent lamps is produced from high purity tungsten oxide doped with an alkaline metal, for example, in the form of an alkaline silicate. As used herein, the term "doped" means the intentional addition of an impurity in a very small controlled amount to improve properties such as creep-resistance in articles formed from the doped material. For example, one well known dopant is comprised of potassium disilicate and aluminum chloride and is the dopant referred to in the following background discussion. Doped tungsten oxide powder is reduced to tungsten metal powder containing traces of the dopant within the individual grains of powder. Residues of the dopants remaining on the surface of the reduced powder particles are removed by acid washing, for example, with hydrochloric and hydrofluoric acid.
The doped metal powder is ram pressed to form an elongated compact with a square-like cross-section having four or more sides. In ram pressing, a uniaxial pressure is applied to the powder, and a highly porous and fragile compact is formed. The compact is heated to about 1200.degree. C. in a presintering operation to impart adequate strength for handling and resistance sintering of the compact. A resistance heating current of about 4000 to 6000 amps is transmitted through the compact for sintering. Such resistance sintering heats the compact to about 2600.degree. to 3000.degree. C. and densifies the tungsten powder compact into an ingot.
During sintering, aluminum and silicon diffuse out of the ingot and evaporate away. Much of the potassium, which is insoluble in tungsten, is retained in the form of particles residing in pores inside the ingot. However, sintering provides a driving force for removal of some potassium from the ingot, and a gradient in potassium concentration between the center and outer surface of the ingot is produced during sintering.
The ingot is elongated by swaging, and drawn into a fine wire in a series of annealing and wire drawing operations. During the swaging and drawing processes, the doping material is distributed in long rows of fine pores or bubbles aligned parallel to the wire axis. The bubbles are maintained during the wire reduction processes and in subsequent high temperature operation because of the insolubility of potassium in tungsten and the vapor pressure of the doping substance. After each wire drawing step, highly deformed grains in the tungsten wire are recrystallized by the intermediate anneals. The rows of bubbles prevent movement of grain boundaries perpendicular to the wire axis during and after recrystallization of the wire.
Tungsten filaments are operated in lamps at temperatures of about 2000.degree. to 3000.degree. C. The filament material must be creep-resistant at such elevated temperatures because of high stresses exerted upon the filament by mechanical or thermal means. Creep distorts coiled filaments, increasing the radiation heat loss and decreasing the luminous efficiency of coiled filaments. Creep in tungsten filaments can also cause individual turns between coiled filaments to contact one another and short out, thereby shortening the life of the filament. In addition, excessive creep can result in premature breakage of the filament.
The rows of potassium bubbles pinning the grain boundaries provides a creep-resistant interlocking grain structure for the tungsten wire resulting in long-life filaments. The absence of the bubbles results in creep from grain boundary sliding, and rapid failure in operation of the filament. It is, therefore, necessary that the filament contain potassium or other material which will produce the bubbles described above, and a uniform distribution of the rows of bubbles for providing uniform creep-resistance throughout the entire length of filamentary wire.
It should be understood that one tungsten ingot can be reduced to about 80 kilometers of filamentary wire, and as a result, small non-uniformities of dopant distribution in the tungsten ingot can lead to insufficient bubbles and excessive creep in localized portions of the wire. The tungsten ingot forming process described above tends to produce non-uniformities in dopant distribution resulting from non-uniformities in initial additions or non-uniform removal of dopant during sintering. This leads to inhomogenities in the wire properties giving rise to localized sagging or creep in the filaments.
The numerous swaging and wire drawing operations reducing the radius of the ingot act to homogenize the radial distribution of dopant in the drawn wire. However, non-uniformities in the length dimension of the ingot are exacerbated by the swaging and wire drawing operations. We have discovered that unidirectional ram pressing produces compacts with density variations of about 10 percent along the length of the compact. There is also variation in the mean compact density from pressing to pressing. Resistance heating and, therefore, temperature varies with changes in density, and as a result, variations in the compact density cause non-uniform heating in the compact during the sintering operation. Such non-uniform heating during sintering results in a non-uniform temperature distribution within the ingot, and a non-uniform distribution of dopant along the length dimension of the ingot.
An object of this invention is a method for forming tungsten ingots having a greater uniformity of density within the ingot, and a greater uniformity of density between separate ingots.
Another object of this invention is a method that reduces the steps performed in forming a tungsten ingot.